A nonwoven fabric made of extra-fine fibers is manufactured from split fibers or with a flash spinning method or a melt blown method, and made use for filter applications. The nonwoven fabric, however, is mainly composed of a resin such as polypropylene, nylon, or polyethylene terephthalate, and hence flame retardancy or heat resistance has been insufficient and use thereof at a high temperature has not been suitable. Though some techniques for manufacturing a nonwoven fabric from fibers composed of a flame retardant polymer have been attempted, such an unfavorable condition as melt fracture or high melt tension takes place in an attempt to obtain extra-fine fibers, and it has been difficult to obtain a nonwoven fabric formed from flame retardant extra-fine fibers with good productivity.
Japanese Patent Laying-Open No. 3-180588 (PTD 1) discloses a nonwoven fabric made of flame retardant polyetherimide (which may hereinafter be referred to as PEI) fibers alone and a composite nonwoven fabric of PEI fibers and inorganic fibers. The nonwoven fabric in PTD 1, however, requires impregnation with a chlorine-based aliphatic hydrocarbon compound such as methylene chloride or trichloromethane for adhesion among the PEI fibers, and use of a solvent may affect characteristics of the PEI fibers. Such a solvent has recently been found to affect human bodies or environments, and development of an alternate technique has been demanded from a point of view of load imposed on environments or cost associated with recovery of the solvent.
As a nonwoven fabric made of PEI fibers, a nonwoven fabric mainly made of PEI fibers having a specific structure and three-dimensionally interlaced with one another has been disclosed (Japanese Patent Laying-Open No. 2012-41644 (PTD 2)). Amorphous PEI is not only high in melting point and excellent in heat resistance owing to its molecular frame but also excellent in flame retardancy, and it is made use of for fibers or engineering plastics. Examples in PTD 2, however, disclose only a nonwoven fabric made with a spun lace method, which has a relatively high fineness with a fiber diameter being 2.2 dtex (corresponding to 15 μm).